Decoupled RFID reader and interrogator

ABSTRACT

A decoupled RFID reader for reading an RFID tag using a single interrogator and a plurality of reader modules. The interrogator transmits an RF signal to supply power for the tag, and also sends commands to the tag. One of the reader modules communicates with the tag to trigger the tag to transmit a tag response including tag identification data. The first one of the reader modules receives the tag response and communicates the tag identification data of the tag to all of the other reader modules.

RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent application Ser. No. 60/712,957, filed Aug. 31, 2006, entitled “RFID Systems And Methods”, the disclosure of which is hereby incorporated by reference.

BACKGROUND

RFID stands for Radio-Frequency IDentification. An RFID transponder, or ‘tag’, serves a similar purpose as a bar code or a magnetic strip on the back of a credit card; it provides an identifier for a particular object, although, unlike a barcode or magnetic strip, some tags support being written to. An RFID system carries data in these tags, and retrieves data from the tags wirelessly. Data within a tag may provide identification for an item in manufacture, goods in transit, a location, the identity of a vehicle, an animal, or an individual. By including additional data, the ability is provided for supporting applications through item-specific information or instructions available upon reading the tag.

A basic RFID system comprises a reader, including an interrogator module (transmitter) and a closely-coupled reader (receiver) module (a transceiver is often used), and a transponder (an RFID tag) electronically programmed with identifying information. Both the reader/interrogator and transponder have antennas, which emit and receive radio signals to activate the tag, read data from the tag, and write data to it.

Several types of RFID tags exist, including ‘active’ and ‘passive’ tags. Active RFID tags are powered by an internal battery, while passive tags operate without an internal battery source, deriving the power to operate from an electromagnetic field typically generated by an interrogator.

The interrogator module in the reader emits an RF activation signal (such as a ‘select’ command) with a range of anywhere from contact to 100 feet or more, depending upon the interrogator's power output, the radio frequency used, the antenna used, and environmental conditions. The RF signal from the interrogator provides power to operate a passive tag's integrated circuit or microprocessor and associated memory.

In a tag-read situation, when an RFID tag passes through the electromagnetic zone created by the interrogator (i.e., when the tag is ‘in-field’), it detects the activation signal, which powers the tag. Upon receiving a ‘read tag’ command from the reader module, the tag conveys its stored data to the reader module, using power provided by the interrogator. The reader decodes the data received from the tag's integrated circuit and the decoded data may be processed by the reader, or passed to another device (e.g., a computer) for processing.

In a tag-write situation, when an RFID tag is ‘in-field’, it detects the interrogator's activation signal, upon which the tag transfers data sent from either the interrogator or the reader module to the tag's internal memory (using a write command), again using power harvested from the transmit signal to power the tag to process the command and provide a response.

Thus, unlike other radio systems, passive RFID systems provide not only communication between elements, but one of the elements (the reader/interrogator) must power the other element (the tag/transponder) in the communication band itself. Communication is composed of tag commands (select, read, write, lock, kill, etc), and tag responses to those commands. Power is provided by the reader in the act of issuing a tag command and receiving a tag response. Note that transmit and receive are discussed from the reader's point of view; from the tag point of view these terms would be reversed.

It is unlikely that for a given reader and tag combination that the transmit communication (tag commands), receive communication (tag responses), and power provided are all optimized. Because tag commands “ride” on the power signal as modulation, these two legs are commonly well matched. However, this is not always the case. The receiver may be getting just enough power, but the specific modulation scheme may have too little signal-to-noise gain to be differentiated from the power, thus making the signal from the tag is unreadable. Conversely, if the signal is detectable, but the power is insufficient for powering a response, the traditional solution has been to use more expensive battery-backed tags (semi-passive tags) to add additional power.

There are also second-order effects in conventional RFID readers, which include closely spaced transmitter and receiver sections. For example, if the tag is not getting sufficient power, turning up the power alone may not achieve the desired result. The additional transmit signal may swamp the receiver section (if there is not sufficient isolation between sections) degrading receive sensitivity and thus reducing range, rather than increasing it. The tag command is often issued at 27-36 dB, while the tag response is often at −40 dB, resulting in a 70 dB difference between the transmit and receive legs (a very high end RFID system could be designed for a 100 dB difference). Each 3 dB is a doubling in power, so 70 dB represents a transmit signal strength that is ˜17 million times greater than the receive signal strength. This second-order effect makes RFID reader design a complex problem, with changes in one area causing problems in another.

PROBLEM TO BE SOLVED

In the case of the prior art, typically either (1) the tag is powered at a greater range (D1) than the range within which the receiver can ‘hear’ the tag response (D2) (so the tag can receive and execute commands from a power module); the tag's returned signal is too weak to be read by the receiver (i.e., the receiver is not sensitive enough); or (2) the receiver is capable of reading the tag beyond the range that the tag response is achieving (i.e., the tag-powering transmitter does not generate sufficient output power for the tag-receiver distance).

By attempting to ‘optimize’ the system to have D1=D2, the performance of the system is constrained to the lower value of D1 or D2, which results in sub-optimal system performance.

Additional isolation between transmit and receive signals is typically accomplished by increasing the size of the reader or by adding isolation elements (different feed paths, circulators) which will add size and expense to an RFID reader configuration. In many RFID systems, isolation of only −25 to −40 dB can be achieved between transmit and receive signals between the closely-coupled transmit and receive sections.

SOLUTION TO THE PROBLEM

A decoupled RFID reader is disclosed for reading an RFID tag using a single interrogator and a plurality of reader modules. The interrogator transmits an RF signal to supply power for the tag, and also sends commands to the tag. One of the reader modules communicates with the tag to trigger the tag to transmit a tag response including tag identification data. The first one of the reader modules receives the tag response and communicates the tag identification data of the tag to all of the other reader modules.

The present system decouples the power plus transmit function from the receiver to allow a higher powered transmit signal to power the tag without degrading the receiver. Alternatively, decoupling may include separating the power transmission function from the transmit plus receive function. Decoupling the two functional sections as different modules can achieve isolations well below the thermal noise floor in the −110 dB range.

Full separation of power from transmit from receive may be accomplished by using three separate modules, i.e., a power transmitter, a command transmitter, and a read-only receiver.

In summary, in most radio systems, read range is a function of the transmit link quality and the receive link quality. RFID passive tag operations add in the complication of the power needs of the tag. Decoupling the power needs from the other issues allows optimization of the RFID reader. The present system thus allows a plurality of receive-only reader modules to be communicatively associated with a single interrogator, and outperform an equivalent traditional reader.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high-level diagram of an exemplary embodiment of an interrogator and two types of reader modules that may be used by the present system;

FIG. 2A is a diagram of an exemplary embodiment of the present system, showing multiple connected reader modules associated with a single interrogator via an RFID tag;

FIG. 2B is a diagram of an exemplary alternative embodiment of the present system, including one or more reader modules, and a separate tag power transmitter;

FIG. 2C is a diagram of an exemplary alternative embodiment of the present system, employing separate tag power, command transmit, and receive modules;

FIG. 3 is a flowchart showing an exemplary set of steps performed in operation of the present system;

FIG. 4 is a flowchart of an exemplary embodiment of the present method, showing a set of steps performed by the method and associated system in a real-time location system application; and

FIG. 5 is an exemplary diagram showing the delta distance between D1, the range at which communication between a reader and a passive tag can be established, and D2, the distance at which the tag has enough power to operate.

DETAILED DESCRIPTION

The present system physically separates the transmitter (which provides both tag power and tag command functions) and receiver elements of a traditional RFID reader to increase isolation and hence provide higher receiver sensitivity, using a method to time-coordinate the transmit and receive functions. In another embodiment, the power transmission element is additionally separated from the transmit and receive elements.

Time coordination is typically performed by a real time clock on each of the readers coupled with either 1) periodic synchronization to a known absolute time base (NTP, GPS), or 2) periodic synchronization to a known common time base (an epoch value sent out by a system element, such as a reader).

FIG. 1 is a high-level diagram of an exemplary embodiment of an interrogator and two types of reader modules that may be used with the present system. As shown in FIG. 1, interrogator 101 includes a transmitter 104 coupled to, and controlled by, processing logic 106. Interrogator 101 sends an RF transmission to energize RFID tag 203 (and other tags within the interrogator's range), and also transmits modulated tag commands to one or more RFID readers.

Receive-only reader module 102 includes a receiver 105 coupled to, and controlled by, processing logic 106. Reader module 103 with transceiver 109 includes a receiver 105 and a transmitter 108, both of which are coupled to, and controlled by, processing logic 106. Transmitters 104/108, and receiver 105 are each connected to an antenna 107, although, in an alternative embodiment, a transmitter and receiver pair may share the same antenna.

FIG. 2A is a diagram of an exemplary embodiment of the present decoupled reader/interrogator system 200(A), where “decoupled” includes separating the power+transmit function from the receiver to allow a higher powered transmit signal to power an RFID tag without degrading the receiver's performance. Alternatively, decoupling may include separating the power transmission function from the transmit+receive function. From a practical standpoint, the distance between the interrogator and the associated readers should be sufficient to achieve at least 40 dB of isolation. The present system is also ‘decoupled’ to the extent that at least one of the readers 102 may be located significantly further from a target tag, e.g., tag 203, than the interrogator 101, and is capable of reading the tag from that particular location.

As shown in FIG. 2A, decoupled system 200(A) comprises an interrogator 101 and one or more separate receive-only reader modules 102(*). In this embodiment, interrogator 101 transmits a signal to power the readers 102(*), and also transmits modulated tag commands to the readers. Each receive-only reader includes two receive chains—one that processes the transmit signal in approximately the 0 to 36 dB range and one that processes the tag response in approximately the −30 to −75 dB range. Operating receivers must lock on to the transmit signal, which can be accomplished by coordinating time with the transmitter or a common time base, or using phase information and a phase locked loop.

In an exemplary embodiment, system 200A includes multiple networked or otherwise communicatively connected read-only reader modules 102(*) associated with a single interrogator 101, for reading an RFID tag 203. As used herein, an asterisk in parentheses following a reference number indicates an arbitrary one of the type of entity designated by the reference number.

As indicated in FIG. 2A, a single interrogator 101 and a plurality of receiver modules 102(*), 103(A) are interconnected by a network (wired or wireless) indicated by dotted lines 211, 212, 213, and 214. Each of the receiver modules 102(*), 103(*), as well as the interrogator 101, in the network is also communicatively coupled (as indicated by dotted lines 230, 231, 232, 233) to a coordinating reader/processor 204, which may include the read/write functions of readers 102 or 103, or which may, alternatively, function solely as a processor which coordinates the activities of the reader modules. System 200A thus comprises, in effect, a distributed reader including N 102(*) read-only modules and a separate interrogator 101, that functions as one logical reader.

In the embodiment described in FIG. 2A, each reader 102(*) is located in sufficient proximity to tag 203 so that the reader can read information from (and optionally write information to) the tag (as shown by arrows 221, 222, 223) when interrogator 101 is transmitting a tag power (activation) signal 210.

FIG. 2B is a diagram of an exemplary alternative embodiment 200(B) of the present system, including one or more reader transceiver modules 103(*), each of which outputs power only in the reader's/tag's frequency band. One or more of the readers 103(*) sends a modulated carrier and receives the tag response. A separate tag power transmitter 201 sends a carrier or broadband noise to power the tag 203.

FIG. 2C is a diagram of an exemplary alternative embodiment 200(C) of the present system, employing separate tag power transmit, command transmit, and read-only receive modules. As shown in FIG. 2C, system 200(C) comprises a single tag power transmitter 201, a tag command transmit module 110, and one or more read-only receive modules 102(*).

Each of the readers 102/103 may coordinate either using a direct protocol (a reader-to-reader protocol) or via a host system (not shown). In an exemplary embodiment, a first one of the readers may handle all of the singulation process and then hand off tag data to a second reader. Singulation is a method by which an RFID reader identifies a tag with a specific serial number or other characteristic from a number of tags in its field. RFID readers typically use an anti-collision protocol to communicate with multiple tags in a reader's field. Singulation enables RFID readers to scan multiple tags simultaneously. To ensure that tag signals do not interfere with one another during the scanning process, the singulating reader first ascertains what tags are present, and then addresses the tags individually.

FIG. 3 is a flowchart of an exemplary embodiment of the present method, showing a high-level set of steps performed by the method and associated systems 200(A), 200(B) and 200(C). As shown in FIG. 3, at step 305, interrogator 101 energizes the field of a target tag (e.g., tag 203) in response to request from an initiating reader, e.g., reader module 103(A), or coordinating reader/processor 204. The initiating reader then waits for a tag response.

At step 307, the receiving readers build a carrier image of what they expect the tag transmission to be (if necessary). The tag command and the tag response are both effected via modulation applied to the transmit carrier. To extract a tag response, a receiver subtracts the carrier from the complete tag response (the modulated carrier) leaving only modulation behind, which is the tag response itself.

At step 310, on receiving one or more tag responses, the initiating reader performs singulation to select and communicate with a single responding tag via the standard tag protocol for that particular type of tag. Next, at step 315, the initiating reader communicates the tag ID of selected tag to all other coordinating readers (e.g., readers 102(B) and 102(C) in FIG. 2A) coordinating with the initiating reader, in case the other readers receive signals from tags that the initiating reader did not singulate out.

On transmit, the tag command signal plus carrier is the modulated tag signal which is transmitted to a tag. On receive, the modulated tag response signal minus the carrier is the tag response signal. Therefore, the receiver needs to know the carrier characteristics to extract the tag response signal.

At step 320, the tag response and optional additional information about the response [e.g., noise profile data such as bit error rate (BER), signal strength, and/or signal-to-noise ratio (SNR)] are sent (via the reader network) to coordinating reader/processor 204 from the coordinating readers receiving tag signals. Coordinating reader/processor 204 forwards the response to a central processing facility (not shown), and/or processes the additional data. Coordinating reader/processor 204 may be one of the coordinating readers, or it may, alternatively, be a non-tag-reading processing device.

At step 325, the noise profile data is sent back to the coordinating readers to allow tuning of software filters or other software mechanisms. Noise modeling is most useful in adaptive filtering to remove the modeled noise allowing extraction of even lower energy signals than can be achieved without removing (or incorrectly removing) the noise in processing logic 106 to improve reader performance.

FIG. 4 is a flowchart of an exemplary embodiment of the present method, showing a set of steps performed by the method and associated system in a real-time location system (RTLS) application. An RTLS may be constructed for determining two-dimensional tag position utilizing a minimum of three fixed read-only modules 102 and a single interrogator 101, which may either be stationary or moving. An RTLS for determining three-dimensional tag position requires a minimum of four fixed read-only modules 102. As an example, a handheld reader that reads and writes may be employed as the interrogator unit 101. Passive read-only readers 102(*) at known locations in the room may then pick up transmissions between the handheld reader and a tag 203, and use the tag response to triangulate the tag location. Common timing is also needed between the readers to correlate the ranges to calculate the tag location.

In an exemplary embodiment, common timing is provided as follows. Each reader reports back to the RTLS processor the following data every time the reader interacts with a given tag: the tag ID, the noise value (e.g., bit error rate), and a timestamp.

The RTLS system takes the reports coming from each reader and splits them up by tag ID so that, for example, all reports from all readers for tag ID 1 are together. The timestamp is then used to associate the reports from the readers at or near the same point in time (e.g., all of the reports from tag ID 1 are received at exactly noon from all the readers). An alternative is to have the RTLS system keep the only good time, and query each reader for its last saved set of tag data. In this case, each reader keeps an epoch time (time since it recorded the data from the tag) and reports that as the time. Since the RTLS system knows what time it made the request, it can estimate the time of the tag read. In a more sophisticated alternative embodiment, estimation can be used to fit reports received at slightly different times to an average time.

The readers all determine the bit error rate (BER) from the tag. If the noise environment at each reader is similar (an assumption that can be improved with statistical noise modeling), then any differences in BER from a common baseline can be assumed to be due to range. This technique can be improved by calibrating the system against a test tag in a known position to correlate the relationship between BER and range. This BER difference per reader is then converted to a distance. With multiple distances to the tag from multiple readers, triangulation, or other geometric methods, or least squares, or other statistical methods, can be used to determine a tag position.

As shown in FIG. 4, at step 405, timing data for each reader-to-tag leg is determined, if the RTLS is to utilize timing information to determine a target tag's location. The return signal strength or other function that correlates with distance, such as bit error rate or signal strength, can be used to estimate tag to reader distance, as indicated above.

At step 410, either (1) noise profile data, such as BER, signal strength, and/or SNR, or (2) time of arrival data is converted to reader-to-tag ranges, either by processing logic 106 in each receiving reader or in coordinating reader/processor 204. Then, at step 415, the tag-to-reader distances determined with respect to information of type (1) or (2) above are combined with known receiver (reader module 102/102) positions and converted to a tag position relative to any desired component in system 200 whose position is known. Multiple tag-to-reader distances can be combined to determine tag location using mechanisms such as least squares regression, Kalman filtering, or geometric methods such as triangulation.

FIG. 5 is an exemplary diagram showing the delta distance (D2−D1) between D1, the range at which communication between a reader (transceiver) and a passive tag can be established; and D2, the distance at which the tag has enough power to operate, i.e., to receive and execute commands from the transceiver.

In the case of the prior art, typically either (1) the tag is powered at a greater range than the range within which the receiver can ‘hear’ the tag response, but the tag's returned signal is too weak to be read by the receiver (i.e., the receiver is not sensitive enough); or (2) the receiver is capable of reading the tag beyond the range that the tag is achieving sufficient power (i.e., the tag-powering transmitter does not generate sufficient output power for the tag-receiver distance). FIG. 5 shows case (1).

By attempting to ‘optimize’ the system to have D1=D2, the performance of the system is constrained to the lower value of D1 or D2. By decoupling the reader elements, in accordance with the present system, the system can be optimized for the greater value of D1 or D2.

Each of the embodiments described above constitute a distributed reader system composed of multiple, relatively low cost components acting as a single, more-capable reader. Certain changes may be made in the above methods and systems without departing from the scope of that which is described herein. It is to be noted that all matter contained in the above description or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. For example, the methods shown in FIGS. 3 and 4 may include steps other than those shown therein, and the systems shown in FIGS. 1, 2A, 2B, and 2C may include different components than those shown in the drawings. The elements and steps shown in the present drawings may be modified in accordance with the methods described herein, and the steps shown therein may be sequenced in other configurations without departing from the spirit of the system thus described. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method, system and structure, which, as a matter of language, might be said to fall there between. 

1. A decoupled RFID reader for reading an RFID tag comprising: a single interrogator; and a plurality of reader modules; wherein the interrogator transmits an RF signal to supply power for the tag, and also sends commands to the tag; wherein a first one of the reader modules communicates with the tag to trigger the tag to transmit a tag response including tag identification data and wherein the first one of the reader modules receives the tag response and communicates the tag identification data of the tag to all of the other reader modules.
 2. The decoupled RFID reader of claim 1; wherein the tag response includes noise profile data sent from each of the plurality of reader modules to a coordinating reader/processor; and wherein the coordinating reader/processor processes the noise profile data and sends processed data back to the reader modules to allow tuning of software filters to improve reader performance.
 3. The decoupled RFID reader of claim 2, wherein the noise profile data includes at least one data type selected from the data types consisting of bit error rate, signal strength, and signal-to-noise ratio.
 4. The decoupled RFID reader of claim 1, wherein the single interrogator and the plurality of reader modules function as a single logical reader.
 5. The decoupled RFID reader of claim 1, wherein all tag-write functions are performed by the interrogator, and all tag-read functions are performed by certain ones of the plurality of reader modules.
 6. The decoupled RFID reader of claim 1, wherein the first one of the reader modules communicates with a plurality of said RFID tags to singulate one of the tags; and wherein the first one of the reader modules receives the tag response from a singulated tag, and communicates the tag identification data of the singulated tag to other reader modules.
 7. The decoupled RFID reader of claim 1, wherein: a second plurality of the RFID reader modules are located in known positions, relative to each other; noise profile data is sent from each of the second plurality of the reader modules to a coordinating reader/processor; the tag distance from each reader is determined by the coordinating reader/processor using the noise profile data; and a two-dimensional tag position is determined when said second plurality is equal to three, and a three-dimensional tag position is determined when said second plurality is equal to four, each said tag position being determined by the tag distance from each reader and use of a coordinated time base.
 8. The method of claim 7, wherein the noise profile data includes at least one of the information types consisting of bit error rate, signal-to-noise ratio, and signal strength.
 9. A decoupled RFID reader for reading an RFID tag comprising: a single interrogator; and a plurality of reader modules, each including a transceiver; wherein the interrogator transmits an RF signal to supply power for the tag; wherein more than one said transceiver in the plurality of the reader modules outputs a command, comprising an RF signal in the communication band of the tag, and receives data from the tag in response to the command; and wherein one of the reader modules receiving the data in a tag response communicates the data from the tag to a plurality of the other reader modules, which then use this data to allow future communication with the tag.
 10. The decoupled RFID reader of claim 9; wherein the tag response includes noise profile data sent from each of the plurality of reader modules to a coordinating reader/processor; and wherein the coordinating reader/processor processes the noise profile data and sends processed data back to the reader modules to allow tuning of software filters to improve reader performance.
 11. The decoupled RFID reader of claim 10, wherein the noise profile data includes at least one data type selected from the data types consisting of bit error rate, signal strength, and signal-to-noise ratio.
 12. The decoupled RFID reader of claim 9, wherein: a second plurality of the RFID reader modules are located in known positions, relative to each other; noise profile data is sent from each of the second plurality of the reader modules to a coordinating reader/processor; the tag distance from each reader is determined by the coordinating reader/processor using the noise profile data; and a two-dimensional tag position is determined when said second plurality is equal to three, and a three-dimensional tag position is determined when said second plurality is equal to four, each said tag position being determined by the tag distance from each reader and use of a coordinated time base
 13. The method of claim 12, wherein the noise profile data includes at least one of the information types consisting of bit error rate, signal-to-noise ratio, and signal strength.
 14. A decoupled RFID reader for reading an RFID tag comprising: a first transmitter; a second transmitter; and a plurality of read-only reader modules; wherein the first transmitter transmits an RF signal to supply power for the tag; wherein the second transmitter transmits commands to the tag; wherein the second transmitter communicates with the tag to trigger the tag to transmit a tag response including tag identification data; and wherein the first one of the reader modules receives the tag response and communicates the tag identification data of the tag to all of the other reader modules.
 15. The decoupled RFID reader of claim 14; wherein the tag response includes noise profile data sent from a plurality of the reader modules to a coordinating reader/processor; and wherein the coordinating reader/processor processes the noise profile data and sends processed data back to the reader modules to allow tuning of software filters to improve reader performance.
 16. The decoupled RFID reader of claim 15, wherein the noise profile data includes at least one data type selected from the data types consisting of bit error rate, signal strength, and signal-to-noise ratio; wherein the first one of the reader modules receives the tag response from a singulated tag, and communicates the tag identification data of the singulated tag to other reader modules.
 17. The decoupled RFID reader of claim 14, wherein: a second plurality of the RFID reader modules are located in known positions, relative to each other; noise profile data is sent from each of the second plurality of the reader modules to a coordinating reader/processor; the tag distance from each reader is determined by the coordinating reader/processor using the noise profile data; and a two-dimensional tag position is determined when said second plurality is equal to three, and a three-dimensional tag position is determined when said second plurality is equal to four, each said tag position being determined by the tag distance from each reader and use of a coordinated time base.
 18. The method of claim 17, wherein the noise profile data includes at least one of the information types consisting of bit error rate, signal-to-noise ratio, and signal strength.
 19. The decoupled RFID reader of claim 14, wherein: at least four of the plurality of RFID reader modules are located in known positions, relative to each other; and a three-dimensional tag position is determined using the tag distance from each reader and a coordinated time base.
 20. A method for reading an RFID tag comprising: sending a signal from a single transmitter to energize the tag, which sends a response to a plurality of networked RFID readers.
 21. The method of claim 20, wherein each of the plurality of networked RFID readers is a read-only reader.
 22. The method of claim 20, wherein the response includes a tag ID.
 23. The method of claim 20, including communicating the response, from each of the readers receiving the response, to a single coordinating reader/processor.
 24. The method of claim 20, including sending noise profile data, from each of the readers receiving the response, to a single coordinating reader/processor.
 25. The method of claim 24, wherein: the coordinating reader/processor processes the noise profile data for each of the readers sending noise profile data, and sends processed noise profile data back to each respective one of the readers; and the noise profile data received by each of the readers is used to tune at least one software filter in each said reader.
 26. The method of claim 20, wherein: a second plurality of the RFID reader modules are located in known positions, relative to each other; noise profile data is sent from each of the second plurality of the reader modules to a coordinating reader/processor; the tag distance from each reader is determined by the coordinating reader/processor using the noise profile data; and a two-dimensional tag position is determined when said second plurality is equal to three, and a three-dimensional tag position is determined when said second plurality is equal to four, each said tag position being determined by the tag distance from each reader and use of a coordinated time base.
 27. The method of claim 26, wherein the noise profile data includes at least one of the information types consisting of bit error rate, signal-to-noise ratio, and signal strength.
 28. The method of claim 20, wherein: a second plurality of the RFID reader modules are located in known positions, relative to each other; noise profile data is sent from each of the second plurality of the reader modules to a coordinating reader/processor; the tag distance from each reader is determined by the coordinating reader/processor using the noise profile data; and a two-dimensional tag position is determined when said second plurality is equal to three, and a three-dimensional tag position is determined when said second plurality is equal to four, each said tag position being determined by the tag distance from each reader and use of a coordinated time base. 